EP2845920A1 - Oxide compounds as a coating composition - Google Patents

Abstract

The present invention relates to element / element oxide composite structures as a light-absorbing coating, in particular for solar cells, solar collectors and light protection coatings.

Description

Field of the invention

The present invention relates to a coating composition consisting of oxide compounds, a process for producing these oxide compounds and their use.

State of the art

Oxide layers, particularly ceramic and especially alumina (Al ₂ O ₃ ), are used as a coating material for a variety of applications requiring high heat and heat shock stability or resistance to wear, oxidation or hot corrosion, thermal stability and electrical insulation put.

Such layers can act as a diffusion barrier for ions and have high chemical stability and radiation resistance. They are therefore used in many areas. For example, aluminum oxide serves as an insulating material in the field of microelectronics. Because of its chemical resistance and biocompatibility, it is also used in the field of medicine. Coatings with oxides are a good way to protect surfaces against, for example, oxidation or molten salt corrosion. This high chemical stability coupled with very favorable mechanical properties makes oxides an ideal material for protective coatings.

The preparation of suitable oxide compounds is a great challenge, especially the production of suitable oxide compounds with high homogeneity and purity is difficult. For example, alumina is present both as an amorphous phase and in various crystalline modifications with different properties. The latter have the more advantageous properties for protective coatings since amorphous phases are usually softer. Crystalline alumina can exist in various modifications, of which only α-Al ₂ O ₃ (corundum) is thermodynamically stable. The other so-called transition aluminas, such as γ, δ, η, θ, χ, χ'-Al ₂ O ₃ and Al ₂ O ₃ -KII, are metastable and can be converted irreversibly into α-Al ₂ O ₃ . Above 1200 ° C, corundum is the only stable modification. Corundum is also the hardest modification of alumina. The low ionic conductivity and its high thermodynamic stability make it an important coating against oxidation.

A number of methods are known in the art for producing coatings and films of alumina, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), hydrothermal synthesis , sputtering, or sol-gel deposition. Method.

US 6,521,203 describes the preparation of α-Al ₂ O ₃ by calcination of aluminum hydroxide produced by hydrolysis of aluminum isopropoxide, at a temperature of 700 to 1300 ° C. However, this method does not allow the production of thin coatings.

US 5,302,368 describes the preparation of coatings by applying a dispersion of aluminum hydroxide and / or a transition alumina in an aqueous medium. After adjustment of the suspension and spray drying, the dry powder is calcined at 1100 ° C to 1500 ° C in the presence of a chlorine-containing substance.

For industrial applications for deposition of oxide layers, chemical vapor deposition (CVD) is usually used at high temperatures, typically around 1000 ° C, since this technique offers the possibility of coating even complex geometries with well controllable thickness of the coating.

US 5,654,035 describes such a process wherein the body to be coated is contacted at high temperature with a hydrogen carrier gas containing one or more aluminum halides and a hydrolyzing or oxidizing agent. Additionally describes US 6,713,172 the application of this method for coating cutting tools, again at high temperatures of about 1000 ° C.

US 7,238,420 describes a nanotemplate of relatively pure and completely crystalline α-Al ₂ O ₃ on a metal alloy. In the disclosed manufacturing process, crystalline α-Al ₂ O _{3 is} generated directly on the surface of the alloy by CVD. For this purpose, it is pretreated before deposition with a CO ₂ / H ₂ mixture at high temperatures of 1000 ° C to 1200 ° C.

All procedures described require high temperatures. These not only limit the possible substrates, but can also lead to thermal cracks in the coating. Thus, the oxide coatings and the substrate often have different thermal expansion coefficients of film and substrate, resulting in thermally induced cracks in the coating.

In order to avoid the high temperatures, great efforts have been made to develop processes which make it possible to deposit oxide layers at lower temperatures, for example by physical vapor deposition (PVD).

US 5,683,761 describes a method for depositing α-Al ₂ O ₃ using electron beam PVD. However, the substrate must be heated to about 1000 ° C. The deposition of pure oxide, in this case α-Al ₂ O ₃ , thus requires high temperatures.

Variants of the CVD methods, such as plasma assisted / enhanced chemical vapor deposition (PACVD / PECVD) or metal organic chamical vapor deposition (MOCVD) also offer the possibility of using lower temperatures.

So describes Pradhan et al. (Surf Coat, Tech 176 (2004) 382-384 ) that the use of organometallic aluminum compounds leads to the formation of crystalline alumina at low temperatures (higher than 550 ° C). At lower temperatures, only amorphous films were obtained.

Although MOCVD processes offer many advantages, such as lower temperatures, simple processes, uniform coatings or the use of a single precursor, but they also lead to carbonaceous impurities in the coating.

The degree of crystallinity and the crystalline phases within the deposited oxide layer are very important for their mechanical properties. A pure phase with high thermal and mechanical stability is clearly preferred over a mixture of different phases. However, a suitable heat treatment of the coated substrate is necessary, which not only leads to the transformation into the desired phase, but in addition to a densification of the coating, which also plays a major role for the mechanical stability of the layer. Such heat treatment often requires temperatures above 1200 ° C, which are not suitable for many substrates.

In order to avoid the heating of the entire coated substrate, a local heat treatment is recommended. In this context, lasers for the treatment of such ceramic materials have already been used successfully (laser sintering). The coating is heated in a small area by means of a laser beam. Especially in the field of oxide ceramics, these methods are used because they absorb in the field of CO ₂ laser used. A particular problem is the formation of thermally induced jumps in the reconsolidation and cooling of the material. They result from the brittleness of the ceramics and from the high temperature gradient between the impact area and surrounding material, and the different thermal expansion coefficients of the coating and substrate.

So describe Triantafyllids et al. (Appl., Surf, Sci.186 (2002) 140-144 ) and WO 2007/102143 the occurrence of thermal induced cracks during laser sintering. Of course, such defects affect the density and stability of the coating and the homogeneity of the phase transformation.

These effects can be reduced by adding binders to the oxide compounds, for example the aluminum oxide particles. So describes US 6,048,954 such a binder composition for refractory inorganic particles. Although such binders increase the compaction of the coating, they are only applicable to powdered materials and the binder and its residues must be removed after laser sintering or even remain in the oxide layer.

Since the efficiency of laser sintering depends greatly on the absorption of the material to be sintered, absorption is an important criterion. The binder can also contribute to the absorption. So describes US 6,007,764 the use of a mixture of absorbent and ceramic particles to improve absorption. Zheng et al. (Mat. Lett. 60 (2006) 1219-1223 ) use polystyrene coated alumina particles to optimize absorption for CO ₂ lasers. These particles showed better absorption and thus more uniform heating and a reduced temperature gradient. However, organic material always remains as a residue in the coating.

DE 10 2006 013 484 A1 describes the fabrication of an element / element oxide composite, that is, a material containing element and the corresponding element oxide, in this case, metal core and oxide sheath nanowires.

The disadvantage of most processes for the production of oxide layers is the high temperatures of the process. at the laser sintering method, the main drawbacks are that only very specific lasers in a particular wavelength range, usually CO ₂ laser, are suitable to be used, the precursor used does not absorb other wavelengths. This causes high temperature gradients and leads to a higher load on the substrate and to thermally induced cracks and defects. Therefore, the addition of additional binders is often necessary to increase the absorption of the laser energy and to achieve a high quality of the coating. However, residues of these binders remain in the coating. Also, the production of high-quality and defect-free coatings requires a high degree of experience, since an influence on the underlying substrate or an excessive heating must be avoided.

task

The present invention has for its object to overcome the disadvantages of the prior art in the preparation of oxide layers as a coating composition. The object of the invention is, in particular, to provide a process which enables the preparation of suitable oxide compounds as a coating composition.

solution

This object is achieved by the invention with the features of the independent claims. Advantageous developments of the inventions are characterized in the subclaims. The wording of all claims is hereby incorporated by reference into the content of this specification. The invention also includes all reasonable and in particular all mentioned combinations of independent and / or dependent claims.

In the following, individual process steps are described in more detail. The steps do not necessarily have to be performed in the order given, and the method to be described may also have other steps not mentioned.

1. a) Aufbringen einer Element/Elementoxid-Kompositstruktur auf dem Substrat
2. b) (Kurzes) lokales Erhitzen der Element/Elementoxid-Kompositstruktur, vorzugsweise mittels eines Lasers.

To achieve the stated object, a method for producing oxide layers is proposed, which comprises the following method steps:

1. a) applying an element / element oxide composite structure on the substrate
2. b) (Brief) local heating of the element / element oxide composite structure, preferably by means of a laser.

By the method according to the invention, surprisingly, oxide layers are obtained which have few to no defects and high hardness and densification. The composite structure is called a composite structure because it consists of both the element and element oxide.

The inventive coating of the surface with the element / element oxide composite structure is preferably carried out by the metal organic chamical vapor deposition (MOCVD) method. In this process, organometallic precursors (precursors) are converted into the gas phase and thermally decomposed, the non-volatile decomposition product usually being deposited on or on the substrate. The precursors used in the invention have the general formula

El (OR) _n H ₂

wherein El is Al, Ga, In, Tl, Si, Ge, Sn, Pb or Zr and R is an aliphatic or alicyclic hydrocarbon radical and n is 1 or 2.

The aliphatic and alicyclic hydrocarbon radical is preferably saturated and has, for example, a length of 1 to 20 carbon atoms. Preferred are alkyl or unsubstituted or alkyl-substituted cycloalkyl. The alkyl radical preferably has 2 to 15 C atoms, preferably 3 to 10 C atoms, and may be linear or branched, with branched alkyl radicals being preferred. Examples which may be mentioned here are ethyl, n-propyl, n-butyl and the corresponding higher linear homologues, isopropyl, sec-butyl, neo-pentyl, neohexyl and the corresponding higher isoalkyl and neoalkyl homologues or 2-ethylhexyl. The alicyclic rings may comprise one, two or more rings, each of which may be substituted with alkyl. The alicyclic radical preferably has 5 to 10, particularly preferably 5 to 8, C atoms. Examples include cyclopentyl, cyclohexyl, methylcyclohexyl, norbornyl and adamantyl.

According to the invention, preference is given to using oxide compounds which form ceramic oxides.

Particularly preferred are Aluminiumalkoxydihydride having branched alkoxy having 4 to 8 carbon atoms, in particular aluminum tert-butoxydihydrid. The preparation of such compounds is described in DE 19529241 A1 described. They may be obtained, for example, by reacting aluminum hydride with the corresponding 1: 1 molar ratio alcohol, the aluminum hydride being prepared in situ by reaction of an alkali aluminum hydride with an aluminum halide. Furthermore, the preparation of such compounds from Veith et al. (Chem. Ber. 1996, 129, 381-384 It is also shown that the compounds of formula El (OR) H _{2 may} also comprise dimeric forms.

The compounds are preferably converted to the gas phase and thermally decomposed, the non-volatile decomposition product usually being formed on or on a substrate in the form of the element / element oxide composite structure. Suitable substrates for applying the coating are all customary materials, for example metal, ceramics, alloys, quartz, glass or glass-like, preferably inert to the starting and end products. The thermolysis can be carried out, for example, in an oven, on an inductively heated surface or on a surface located on an inductively heated sample carrier. In inductive heating only conductive substrates, such as metals, alloys or graphite can be used. For substrates with low conductivity, an electrically conductive substrate carrier or oven should be used with inductive heating. The substrate may therefore be both a surface of the reaction space and a substrate placed therein. The reactor space used may have any shape and consist of any conventional inert material, such as Duran- or quartz glass. Reactor rooms with hot or cold walls can be used. The heating can be done electrically or by other means, preferably by means of a high-frequency generator. The oven, as well as the substrate carrier can have any shapes and sizes according to the type and shape of the substrate to be coated, the substrate may for example be a plate, planar surface, tubular, cylindrical, cuboid or have a more complex shape.

It may be advantageous to rinse the reactor space several times with an inert gas, preferably nitrogen or argon, before introducing the precursor. In addition, it may be advantageous to apply an intermediate vacuum if necessary, in order to render the reactor space inert.

Furthermore, it can be advantageous to heat the substrate to be coated, for example metal, alloy, semiconductor, ceramic, quartz, glass or glass-like, to above 500 ° C. before the introduction of the organometallic precursor in order to clean the surface.

The desired element / element oxide composite structure is preferably formed at temperatures of above 400 ° C, more preferably above 450 ° C. Preference is given to temperatures of not more than 1200 ° C, especially not more than 600 ° C, e.g. 400 ° C to 1200 ° C, and preferably 450 ° C to 650 ° C, particularly preferably 450 ° C to 600 ° C. The substrate on or at which the thermolysis takes place is accordingly heated to the desired temperature. The generation of the element / element oxide composite structure according to the invention is independent of the substrate material used and its nature.

The (organometallic) compound, or the precursor can be introduced from a storage vessel, which is preferably heated to a desired evaporation temperature in the reactor. Thus, for example, it may be heated to a temperature between -50 ° C. and 120 ° C., preferably between -10 ° C. and 40 ° C. The thermolysis in the reactor chamber is generally carried out at a reduced pressure of 10 ^-6 mbar to atmospheric pressure, preferably in a range of 10 ^-4 mbar to 10 ^-1 mbar, preferably 10 ^-4 mbar to 10 ^-2 mbar. To generate the vacuum, a vacuum pump system can be connected to the reactor on the output side. It can All conventional vacuum pumps are used, preferred is a combination of rotary vane pump and turbomolecular pump or a rotary vane pump. Conveniently, the storage vessel for the precursor is mounted on the side of the reactor space and on the other side of the vacuum pump system.

When the substrate is heated by induction, e.g. square centimeter-sized, electrically conductive metal platelets or foils are arranged as a substrate in a reaction tube of duran or quartz glass. When adapting the dimensions of the apparatus, substrate areas in the range of square decimeters up to several square meters are also possible. On the reaction tube, the supply vessel, which is tempered to the desired evaporation temperature, is connected on the input side to the precursor and, on the output side, a vacuum pump system. The reaction tube is located in a high-frequency induction field, with the aid of which the substrate platelets or foils are heated to the desired temperature. After setting the desired pressure and introducing the precursor, the substrate is covered with the element / element oxide composite structure.

It is advantageous to regulate the flow rate of the precursor with a valve. The valve can be controlled manually or automatically. Depending on the desired thickness of the coating, the duration of the addition of the precursor may be a few minutes to several hours.

By varying one or more process parameters selected from substrate temperature, gas pressure, precursor temperature, precursor flow (amount of precursor introduced per unit time) and evaporation time, the morphology of the element / element oxide composite structure can be controlled.

In a further development, the element / element oxide composite structure obtained can be subjected to a treatment with a mixture, a solution and / or a suspension of organic and / or inorganic substances.

In a further embodiment of the invention, the substrate can be coated only in desired areas with the element / element oxide composite structure, which also limits the treatment by local heating to these areas.

After cooling, the element / element oxide composite is heated locally, more preferably by means of a laser. This process is also called sintering. In this case, the element / element oxide composite structure is converted into the desired element oxide structure. This change may also involve the conversion to one or more modifications of a crystal structure, most preferably the formation of a single modification of the element oxide.

As a particular advantage of the invention, the element / element oxide composite structure has better thermal conductivity than the pure element oxide and thereby results in a reduced temperature gradient during local heating. This reduces the cracks induced thereby.

In an advantageous development of the invention, the elemental component of the element / element oxide composite structure can function as a binder by melting during heating and thus filling out any cracks and pores that may have resulted from the heating in the element / element oxide composite structure. As a result, no addition of a separate binder is necessary, which could lead to undesirable residues.

In an advantageous development of the invention, the element / element oxide composite structure at the place of heating is not completely converted into the corresponding element oxide. By controlling the laser intensity and exposure time, the degree of conversion can be controlled very accurately. This allows the selective production of areas of specific structure and morphology, and thus, for example, the production of nanowires, nanoparticles and fractal surfaces.

In a further advantageous embodiment of the invention, the element / element oxide composite structure is completely converted at the place of heating into the corresponding element oxide. By controlling the laser intensity and exposure time, the degree of conversion can be controlled up to complete conversion. The melting of the metallic component of the element / element oxide composite structure makes it possible to produce particularly defect-free and uniform oxide layers.

Another great advantage of the present invention is the ability to select the wavelength of the laser from a large wavelength range. The fabricated element / element oxide composite structure may be a broad band absorber and thus absorb light from a very broad wavelength range. The wavelength of the laser can range from UV to electromagnetic waves, preferably in the range of 300 nm to 15 μm, more preferably in the range of 500 nm to 11 μm, even more advantageously, but not limited to, lasers with wavelengths of 488 nm. 514 nm, 532 nm, 635 nm, 1064 nm or 10.6 μm. Continuous (CW) or pulsed lasers can be used.

Preferably, the laser energy used is between 1 milliwatt per square centimeter and several watts per square centimeter, preferably between 1 milliwatt per square centimeter and 10 watts per square centimeter, more preferably between 1 mW / cm ² and 5, depending on the wavelength and element / element oxide composite used W / cm ² .

A particular advantage of the invention is the realization of very low penetration depths of the laser. For example, when using a pulsed laser, the penetration depth can be reduced to a range of less than approximately 400 nm, preferably less than approximately 300 nm, particularly preferably less than approximately 200 nm, particularly preferably less than approximately 100 nm. This not only allows the production of very thin layers, but also a special protection of the substrate. The layer thickness of the element oxide layer produced can accordingly be between about 400 nm and about 10 nm, preferably between about 300 nm and about 10 nm, more preferably between about 200 nm and about 10 nm, particularly preferably between about 100 nm and about 10 nm. Theoretically, even the production of only a few monolayers of element oxide would be possible, i. only a few layers of atoms. Furthermore, the low penetration depth protects a temperature-sensitive substrate from thermal energy input and, in addition, mechanical stresses at the interface between coating and substrate are avoided. Thus, it is also possible to use substrates which themselves absorb the laser wavelength used. In addition, only the surface of an element / element oxide composite structure with a greater layer thickness can be converted.

Another particular advantage of the invention lies in the possibility of being able to produce not only particularly thin, but also particularly hard oxide layers, which are particularly preferred offer high corrosion protection due to low permeability.

In a further advantageous development of the invention, the light absorption of the element / element oxide composite structure is measured at the point of the treatment, during the heating or between several sintering processes. By converting the element / element oxide composite structure into the desired element oxide, the absorption, for example of light in the visible region, at the site of heating may change. From this change, it is possible to produce a certain degree of conversion by adjusting process parameters such as but not limited to laser intensity, wavelength, laser exposure time, repetition of heating. After reaching the desired level, the heating can be stopped at this location.

In a particularly advantageous embodiment of the invention, the wavelength of the laser is chosen such that it is reflected by a pure element oxide layer. As a result, after complete conversion of the element / element oxide composite structure at the point of heating, no further absorption of the laser light occurs since the element oxide now present does not absorb it. As a result, the conversion can be stopped "on its own" upon reaching the pure element oxide, as there is no further heating by the laser. This avoids "overheating" of the element oxide layer, which can lead to misalignment, for example, by the formation of grain, in the element oxide layer. In addition, the underlying substrate can be spared in this way. At the same time, this development allows the use of higher laser intensities than in conventional methods with the same layer thickness and substrate were possible.

For this embodiment of the present invention, lasers having wavelengths in the visible range of the light are particularly preferably selected. Due to the absorption properties of the element / element oxide composite structure according to the invention, local heating with lasers with this wavelength range is possible.

Alternatively, the "overheating" of the oxide layer produced can also be used to adjust a certain porosity by the targeted generation of defects.

Another advantage of the present invention lies in the possibility of heating locally, i. not only with the protection of the underlying substrate, but also to perform only in desired areas of the element / element oxide composite structure, for example if such a coating is desired only on the outside of the substrate. On the other hand, it is also possible to treat the entire surface of the substrate, more preferably by line-by-line scanning with the aid of a computer-controlled laser scanner.

Another advantage of the present invention is the ability to create a particular desired structure on the surface of the substrate by targeted proportionate or complete conversion of the element / element oxide composite structure. Due to the possibility of using lasers with shorter wavelengths, structures with a significantly higher resolution than with the usual CO ₂ lasers are possible, theoretically limited by half the wavelength used.

Furthermore, the local heating according to the invention can be carried out with the aid of a computer-controlled laser scanner, preferably with focusing optics, in order to better focus the laser beam.

The present invention further relates to a coating composition, in particular preparable by the abovementioned method according to the invention, comprising oxide layers with high to complete oxide content, which are formed by thermolytic decomposition of organometallic compounds of the formula El (OR) _n H ₂ where El Al, Ga, In, Tl Si, Ge, Sn, Pb or Zr and R is an aliphatic or alicyclic hydrocarbon radical and n is 1 or 2, are prepared at a temperature of more than 400 ° C to form an element / element oxide composite structure and the produced element / element oxide composite structure by brief, local heating, preferably by means of a laser (sintered) is converted into the oxide compound.

The proportion of the oxide compound in the coating composition is preferably at least 80%, preferably at least 95%, particularly preferably almost 100%.

Preferably, the oxide compound is a ceramic oxide, more preferably aluminum or gallium oxide, more preferably alumina and most preferably alumina as α-Al ₂ O ₃ (corundum).

According to the invention, the coating composition may have a high hardness, for example, in the case of alumina, a hardness of about 28 GPa can be achieved.

Furthermore, the coating compositions of the invention are characterized by a high adhesion to the substrate. As a further advantage, the coating compositions according to the invention have a low diffusion coefficient for ions and a low permeability to water. By these properties make them particularly suitable as protection of the substrate against corrosion or wear and abrasion.

Furthermore, the invention relates to the use of the coating composition according to the invention for coating substrates of, for example, metal, semiconductor, alloy, ceramic, quartz, glass or glass-like materials. This represents only a selection of the possible substrates and in no way a limitation. In general, the coating composition according to the invention can be applied to (almost) all substrates. Suitable substrates are known to the person skilled in the art.

The versatility of the element / element oxide composite structure conversion process of the present invention allows numerous applications. According to the invention, very hard, wear-resistant protective layers can be produced for components with high wear and tear. The possibility of producing very defect-free layers makes it possible to use the protective layers for electrical or thermal insulation. Furthermore, applications in the field of medicine, in particular as a coating for implants are possible. Purposefully structured surfaces according to the invention are suitable for example in the field of catalysis, filtration or lithography up to storage media, such as information storage.

Furthermore, the element / element composite structure according to the invention is suitable for the production of surfaces with absorption of a broad wavelength range because of their absorption properties. For example, for Lichtenergieaborbierende coatings for solar cells, sunscreen coatings, solar panels and the like ..

Furthermore, the invention comprises a device for carrying out the local heating, preferably with the aid of a laser, preferably with a computer-controlled laser scanner, particularly preferably with a laser beam focusing optics.

A further advantageous development of the device according to the invention comprises the possibility of measuring the light absorption of the element / element oxide composite structure at the point of the treatment, during the heating or between several sintering processes. This can be done by measuring the intensity of the reflection of the laser at the point of heating or by measuring the intensity of the reflection at the point of heating with another light source of suitable wavelength during sintering or between several sintering operations. This allows complete automation of the method according to the invention.

Further details and features will become apparent from the following description of preferred embodiments in conjunction with the subclaims. In this case, the respective features can be implemented on their own or in combination with one another. The possibilities to solve the problem are not limited to the embodiments. For example, area information always includes all - not mentioned - intermediate values and all imaginable subintervals.

Fig. 1

Aufnahme einer unbehandelten Al/Al₂O₃-Kompositstruktur

Fig. 2

Aufnahme einer behandelten Al/Al₂O₃-Kompositstruktur (5 Watt Laser 5 mm/sec);

Fig. 3

Aufnahme einer behandelten Al/Al₂O₃-Kompositstruktur (10 Watt Laser; 2 mm/sec);

Fig. 4

Absorptionsspektrum einer Al/Al₂O₃-Kompositstruktur (Dicke: 200-400 nm)

Fig. 5

Röntgenbeugungsanalyse (XRD) von verschiedenen Al/Al₂O₃-Kompositstrukturen auf rostfreiem Stahl

Fig. 6

Röntgenbeugungsanalyse (XRD) von verschiedenen Al/Al₂O₃-Kompositstrukturen auf Titan

Fig. 7

Aufnahme einer partiell und mit zunehmender Energie behandelte Al/Al₂O₃-Kompositstrukturen

Fig. 8:

Untersuchung der Wasserpermeabilität von verschiedenen Al/Al₂O₃-Kompositstrukturen

Fig. 9:

Messung der Härte einer erfindungsgemäß hergestellten α-Al₂O₃-Schicht.

XRD analysis: X-ray diffraction analysis

Fig. 1

Image of an untreated Al / Al ₂ O ₃ composite structure

Fig. 2

Recording of a treated Al / Al ₂ O ₃ composite structure (5 watt laser 5 mm / sec);

Fig. 3

Recording of a treated Al / Al ₂ O ₃ composite structure (10 Watt laser, 2 mm / sec);

Fig. 4

Absorption spectrum of an Al / Al ₂ O ₃ composite structure (thickness: 200-400 nm)

Fig. 5

X-ray diffraction analysis (XRD) of various Al / Al ₂ O ₃ composite structures on stainless steel

Fig. 6

X-ray diffraction analysis (XRD) of different Al / Al ₂ O ₃ composite structures on titanium

Fig. 7

Recording of a partially and with increasing energy treated Al / Al ₂ O ₃ composite structures

Fig. 8:

Investigation of the water permeability of different Al / Al ₂ O ₃ composite structures

Fig. 9:

Measurement of the hardness of an α-Al ₂ O ₃ layer produced according to the invention.

The picture series Fig. 1 to 3 clearly shows the influence of the action of the laser as a function of the exposure time, in this case given by the speed with which the laser was moved over the sample.

In detail shows Fig. 1 an untreated Al / Al ₂ O ₃ composite structure before laser treatment. Neither a uniform surface nor a structuring of the surface can be recognized.

Fig. 2 shows an Al / Al ₂ O ₃ composite structure after a short laser treatment. This leads to the formation of new morphologies and structures, in this case nanowires and fractal structures.

Fig. 3 shows a treated to complete conversion Al / Al ₂ O ₃ composite structure. There are only a few defects and the surface appears uniform.

Fig. 4 shows the broad absorption of the Al / Al ₂ O ₃ composite structure (thickness: 200-400 nm). The absorption in a wide wavelength range allows the use of lasers in a wide wavelength range.

Fig. 5 and Fig. 6 show a X-ray diffraction analysis of different Al / Al ₂ O ₃ composite structures treated for different times on two different substrates. It can be clearly seen that the signals of the α-Al ₂ O ₃ crystal structure increase and the signals of the metallic aluminum decrease. This shows an increasing crystallization and formation of α-Al ₂ O ₃ .

Fig. 7 shows the selective conversion in certain range depending on the energy. On the Al / Al ₂ O ₃ composite structure shown, strip-shaped regions with increasing treatment intensity towards the left, which are each separated by untreated strips, were produced according to the method according to the invention. It is clear to recognize the precise resolution and the accuracy with which the inventive method allows the targeted production of structures on surfaces.

Fig. 8 shows the water permeability of differently treated Al / Al ₂ O ₃ composite structures. While the untreated Al / Al ₂ O ₃ composite structure (top, squares) shows a high permeability to water and is therefore unsuitable as corrosion protection, the fully converted Al / Al ₂ O ₃ composite structure according to the invention exhibits no permeability (bottom, triangles) , On the other hand, an Al ₂ O ₃ coating with defects (center, circles) has a significantly higher permeability. This shows how important is a precise control of the conversion conditions to produce tough and safe protective coatings.

FIG. 9 shows the measurement of the hardness of a fully converted Al / Al ₂ O ₃ composite structure by means of nano intention. A hardness of 28 (+/- 2) GPa was measured.

EXAMPLES a) Preparation of the element / element composite structure

The precursor alumuninium tert-butoxide dihydride (Al (tBu) H ₂ ) was deposited in a CVD apparatus under argon at a temperature of 600 ° C on a metal substrate (steel, copper, nickel or platinum) or alternatively on glass or ceramics deposited. The heating of the furnace was carried out inductively, wherein in the case of the glass, a conductive sample holder was used. The pressure in the reactor was about 6.0 × 10 ^-2 mbar. The volatile decomposition products of the precursor (including hydrogen and isobutene) were detected with an attached mass spectrometer. For an Al / Al ₂ O ₃ composite having a layer thickness of about 1 μm, the duration of the precursor flow was about 10 minutes. With longer duration (30 to 90 minutes) higher thicknesses could be obtained. The resulting Al / Al ₂ O ₃ composite structure is dark to black because of its absorption.

b) Local heating

The local heating was carried out by means of a laser. On the one hand, an air-cooled CO ₂ laser with a wavelength of 10.6 μm was used, which is guided by a biconvex ZnSe lens was focused with a focal length of 120 mm. The exposure diameter was 10-12 mm and the conversion width of the laser on the substrate about 20-25 microns. The intensity of the laser was varied between 1 W / cm ² and 5 W / cm ² . This laser is absorbed by the Al / Al ₂ O ₃ composite structure and the alumina layer.

Furthermore, an argon ion laser with wavelengths in the range of visible light was used, which was focused by means of a biconvex lens with the focal length of 120 mm. The exposure diameter was 10-12 mm and the conversion width of the laser on the substrate about 20-25 microns. For the irradiation of the Al / Al ₂ O ₃ composite structure, the wavelengths 514 nm, 488 nm and a wavelength range from 450 nm to 532 nm (mixed line) were used. The intensity was varied between 0.4 W / cm ² and 2 W / cm ² . This laser is absorbed only by the Al / Al ₂ O ₃ composite structure and not by the fully converted aluminum oxide layer.

For brittle substrates, especially some glasses and ceramics, a pulsed laser was used. It was possible to treat thin, and very thin layers of Al / Al ₂ O ₃ composite structure influencing the substrate. For this purpose, lasers with the wavelengths 266 nm, 355 nm, 532 nm or 1064 nm were used. The intensity was kept low and was 200 joules with a pulse length of 4-8 ns. The exposure diameter was 10-12 mm and the conversion width of the laser on the substrate about 20-25 microns. The treatment was carried out with a single pulse as well as a repetition of pulses at a rate of 10 Hz. This allowed a low penetration depth of the laser of only 200-300 nm can be achieved. This allowed the production of very thin oxide layers (<300 nm and even <200 nm) with particularly high corrosion protection and with a hardness of 28 (+/- 2) GPa.

List of quoted literature:

• US 6,521,203
• US 5,302,368
• US 5,654,035
• US 6,713,172
• US 5,683,761
• US 7,238,420
• US 6,048,954
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Claims (6)

Light energy absorbing coatings for solar cells, light protection coatings or solar collectors comprising an element / element oxide composite obtained by the thermolytic decomposition of organometallic compounds of the formula El (OR) _n H ₂ where El is Al, Ga, In or Tl and R is an aliphatic or alicyclic hydrocarbon radical and n is 1 or 2 on a surface at a temperature greater than 450 ° C and at a pressure between 10 ^-4 to 10 ^-1 mbar to form an element / element oxide composite structure. A light-absorbing coating according to claim 1, characterized in that El stands for Al. Light-absorbing coating according to claim 1 or 2, characterized in that aluminum tert-butoxy-dihydride is used as organometallic compound. A light-absorbing coating according to any one of the preceding claims, characterized in that the thermolytic decomposition is carried out at a temperature of between 450 ° C and 650 ° C. Solar cells, light protection coatings or solar collectors comprising a light-absorbing coating according to one of claims 1 to 4. Use of a light-absorbing coating according to one of Claims 1 to 4 for solar cells, light-protective coatings or solar collectors.

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